Controllable low proton source

ABSTRACT

A controllable source of few photons operating at a predetermined wavelength. The source comprises a solid material ( 10 ) having a dilute concentration of elements ( 11 ) implanted therein that emit light at said predetermined wavelength, an excitation device ( 20 ) for exciting said light-emitting elements, and a probe ( 30 ) suitable for capturing, by near field coupling, at least one photon emitted by one of the light-emitting elements. The source is applicable to optical telecommunications.

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/FR01/00402 filed Feb. 12, 2001.

FIELD OF THE INVENTION

The present invention relates to a controllable source of few photonsoperating at a predetermined wavelength.

A particularly advantageous application of the invention lies in thefield of optical telecommunications, in particular high security privatetelecommunication over short distances.

BACKGROUND OF THE INVENTION

In the specification below, the term “source of few photons” is used tomean a light source capable of emitting a single photon or a fewphotons.

In general, conventional optical telecommunications make use ofequipment such as laser sources which are designed to enable photons tobe used at high concentrations, so as to obtain maximum light power andgreatest possible communication distance.

Nevertheless, work is presently under way toward optical communicationssystems using very low photon fluxes, going down to systems using singlephotons (see articles by J.P. Goedgebuer, L. Larger, and D. Delorme,Phys. Rev. Lett., 82, 8, 1656, 1999 and by A. Muller, H. Zbinden, and N.Gisin, Europhys. Lett., 33, 335, 1995). Such few-photon devices areparticularly desired for studying quantum cryptography, which relies onphoton signatures being modified when they are detected, in applicationof Heisenberg's uncertainty principle. It is then necessary to be ableto manipulate and identify single photons, and consequently to havegenuine sources of single photons or to attenuate a flux of photons andwork on statistics.

Presently studied sources of few photons are generally based onrelatively complicated structures: complex III-V structures (referringto the Periodic Table) with microcavities or very dilute chromophoremolecules. However, in any event, such known systems do not give rise tosources of few photons suitable for use in optical telecommunications inorder to deploy quantum cryptography.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a controllable sourceof few photons of predetermined wavelength, the source being simple instructure and having a delivery rate that is low and controllable, andcapable of operating at ambient temperature at wavelengths that are ofinterest for optical telecommunications by fiber, in particular in thenear infrared around 1.5 micrometers (μm).

This and other objects are attained in accordance with one aspect of thepresent invention directed to a source that comprises a solid materialhaving a dilute concentration of elements implanted therein that emitlight at said predetermined wavelength, an excitation device forexciting said light-emitting elements, and a probe suitable forcapturing, by near field coupling, at least one photon emitted by one ofthe light-emitting elements.

Thus, in the source of the invention, photons are emitted bylight-emitting elements implanted in the solid material in a quantitythat is known and controlled as a function of the desired concentration.They are then captured by the probe using the physical processesassociated with near field optics.

To obtain a source of few photons of the invention, the concentration oflight-emitting elements per unit area in the solid material is fewerthan 10 elements per square micrometer (μm²). More particularly, theconcentration per unit area is fewer than 1 per μm² for a single-photonsource.

The controllable source of few photons of the invention thus makes itpossible to implement communications made totally secure by quantumcryptography, whether passing via fiber or via free space without fiber.In the first case, the photon captured by said probe is emitted intofree space and then detected by optical transducers. That type ofimplementation is suitable for short distance communications, of theorder of a few tens of meters (m). In the second case, an optical fiberis coupled to said source to convey the captured photon to a detectordevice. That embodiment can enable communications to be performedbetween sites that are further apart, up to a maximum radius of 20kilometers (km), or within business buildings, for example.

Advantageously, the light-emitting element is a rare earth ion selectedin particular from the list constituted by erbium, praseodymium,neodymium, and ytterbium. More particularly, the Er³⁺ ion is selectedsince its emission wavelength situated at 1.5 μm is in very widespreaduse in optical fiber telecommunications. The erbium ions are preferablyimplanted at a dilute concentration in a solid material presenting avery large band gap, in particular electrical insulators, since it isestablished (Electronics Letters, 25, 11, 718, 1989) that erbiumemission at 1.5 μm and at ambient temperature is obtained underexcitation greater than 0.8 electron volts (eV), which requires hostmaterials in which the band gap is at least equal to said value.

In practice, said probe is formed by a fine point of size smaller than 1μm. By way of example, it is constituted by the end of an optical fibermade of glass or of silica.

Finally, the source of few photons of the invention presents theadvantage of being suitable for being controlled. To this end, provisionis made for it to comprise means for controlling capture by the probe ofthe emitted photon. These means can be means for controlling thedistance between the probe and the light-emitting element.

In conclusion, the controllable source of few photons constituting thesubject matter of the invention opens the way to quantum opticalcommunications systems with or without fiber, over short distances andmade highly secure by the use of quantum cryptography.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description with reference to the accompanying drawing,given as non-limiting examples, explains what the invention consists inand how it can be implemented.

FIG. 1 is a diagram of an embodiment of a controllable source of asingle photon in accordance with the invention.

FIG. 2 is a diagram of another embodiment of a source of few photons inaccordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a controllable single-photon source operating at apredetermined wavelength, e.g. 1.5 μm, the source comprising a solidmaterial 10 or “host” material having light-emitting elements 11implanted therein, said elements emitting at 1.5 μm when they aresubjected to the radiation produced by an excitation device 20. Thepreferred element for emitting light at this wavelength is the Er³⁺erbium ion.

The host material 10 is preferably silica in the form of a chip at iseasy to handle, that is dielectric and therefore has a large band gap,and that is favorable to the rare earth emitting light at ambienttemperature. Silica also presents the advantage of having oxygen as amain ingredient. It has been shown that in order to increase the opticalefficiency of the erbium impurity, it is preferable to use hostmaterials having oxygen or fluorine as main elements (Japanese Journalof Applied Physics, 29, L524, 1990 which is hereby incorporated hereinby reference). In addition, this material is chemically stable, and caneasily be implanted and annealed up to temperatures of 900° C. withoutbeing degraded.

Implantation is performed in compliance with the results of the work ofP. N. Favennec et al. (see “L'implantation ionique pour lemicroélectronilue et l'optoélectronique” [Ion implantation formicroelectronics and optoelectronics], published in “CollectionTechnique et Scientific des Télécommunications” by Editions Mass n, 1993which is hereby incorporated herein by reference).

The parameters proper of implantation and of annealing are conditionedby the choice of silica. The energy of the ions used can lie in therange 10 kiloelectron volts (keV) to 800 keV. The doses implanted mustbe compatible with the desired results. For a single-photon source, theconcentration per unit area of erbium in the solid material 10 shouldpreferably be less than 1 ion per μm². Annealing is necessary toactivate the erbium ions optically, i.e. to ensure that they take up astable site in the host material and in order to rearrange the silicaafter it has been disturbed by the ion bombardment, thereby preventingphotons being absorbed by the resulting defects. The conditions underwhich implanted silica chips are best annealed are as follows: 600° C.to 900° C. in temperature and a few seconds (s) to several tens ofminutes (min) for annealing time.

Erbium is mentioned above specifically because its emission wavelengthat 1.5 μm is most advantageous for optical telecommunications.Nevertheless, other chemical species can be used. Among the rare earths,in addition to erbium, mention can be made of praseodymium (1.3 μm),neodymium (1.06 μm), and ytterbium (1 μm). Mention can also be made ofuranium which emits at 2.5 μm. Finally, light-emitting organic moleculescan also be suitable.

The solid material 10 is not limited to silica, and can naturally beextended to other materials, providing the selected host material andlight-emitting element pair are such that the band gap of the materialis less than the radiative transition energy of the light-emittingelement. The term “radiative transition energy” refers to the energydifferenc between the excited and ground states of the molecule that itradiates when passing from excited state to the ground state. Thisenergy difference is directly related to the wavelength of theradiation.

Apart from silica, other suitable electrically insulating materialsinclude alumina, a nitride, a polymer, a silica or fluorine-containingglass, a fluorine-containing crystal, and a sol-gel (this term refers toa solid gel, i.e. a gel that is applied under a wet form and becomessolid after evaporation). Also suitable are crystalline semiconductors(GaN, GaAs, GaP, GaSb, InP, and derivatives thereof), or non-crystallinesemiconductors such as amorphous or polycrystalline silicon.

The excitation device 20 provides photons at a wavelength shorter thanthe wavelength of the desired light emission. Thus, if it is desired toemit photons at 1.5 μm by exciting erbium ions, the exciting beam shouldcontain photons having a wavelength shorter than 1.5 μm, and can thus beconstituted by a light beam situated in the near infrared, in thevisible, or in the ultraviolet.

Excitation can be performed by any device 20 that can be controlledelectronically using short pulses obtained by photon radiation comingfrom a laser, a source of white light, or by electron bombardment usingan electron gun, for example.

As shown in FIG. 1, a photon emitted by a light-emitting elementcaptured by a probe 30 using the physical mechanism of near fieldcoupling. In general, near field optics (which applies the phenomenon ofnear field coupling) is the result of interaction between a nanometricelement and the total field generated in the vicinity of the lightemitting species, where said interaction takes place at a distance thatis shorter than the wavelength used (“Les ondes évanescentes en optiqueet en optoélectronique” by F. de Fornel, publish in “CollectionTechnique et Scientifique des Télécommunications” by Editions Eyrolles,1997 (which is hereby incorporated herein by reference). In FIG. 1, saidnanometric element is constituted by the probe 30 which is formed by atapering point 31 of size smaller than 1 μm placed at less than 100nanometers (nm) from the surface of the solid material 10. The functionof the probe 30 is thus to capture the photon emitted by thelight-emitting element and to guide it to an optical fiber 40 terminatedon a detector 50.

The tapering point 31 of the probe 30 can be one end of an optical fibermade of silica, fluorine-containing glass, or of silica doped witherbium or any other rare earth. It can equally well be dielectric orsemiconductive, being made of carbon or of silicon. Finally, it can betotally or superficially covered in other materials that are dielectricor metallic.

The light source of FIG. 1 is controlled in intensity by means (notshown) for controlling capture of the emitted photon by the probe 30.These means can be means for controlling the distance between the probe30 and the light-emitting element, such as piezoelectric components,photoelastic components, or microelectromechanical components.

The source of FIG. 2 differs from that of FIG. 1, in that this is asource for delivering a few photons so that the concentration per unitarea of light-emitting elements is higher but still fewer than 10 perμm². Another difference lies in the fact that the photons captured bythe probe 30 are emitted without any fiber into free space until theyreach the detector 50. The range of such a source is naturally less thanthat of a fiber-guided source as shown in FIG. 1.

What is claimed is:
 1. A controllable source of few photons ofpredetermined wavelength, the source being characterized in that itcomprises a solid material (10) having a dilute concentration ofelements (11) implanted therein that emit light at said predeterminedwavelength, an excitation device (20) for exciting said light-emittingelements, and a probe (30) suitable for capturing, by near fieldcoupling, at least one photon emitted by one of the light-emittingelements.
 2. A source according to claim 1, characterized in that theconcentration per unit area of the light-emitting element in the solidmaterial is fewer than 10 per μm².
 3. A source according to claim 2,characterized in that the concentration per unit area is fewer than 1per μm² for a single-photon source.
 4. A source according to claim 1,characterized in that the photon captured by said probe (30) is emittedinto free space.
 5. A source according to claim 1, characterized in thatan optical fiber (40) is coupled to said probe (30) to transport thecaptured photon.
 6. A source according to claim 1, characterized in thatsaid light-emitting element is a rare-earth ion.
 7. A source accordingto claim 6, characterized in that said rare earth ion is selected fromthe list constituted by: erbium, praseodymium, neodymium, and ytterbium.8. A source according to claim 1, characterized in that saidlight-emitting element is uranium.
 9. A source according to claim 1,characterized in that said light-emitting element is an organicmolecule.
 10. A source according to claim 1, characterized in that saidsolid material (10) is an electrical insulator.
 11. A source accordingto claim 10, characterized in that said electrical insulator is selectedfrom the list constituted by: silica, alumina, a nitride, a polymer, aglass, a fluorine-containing crystal, and a sol-gel.
 12. A sourceaccording to claim 1, characterized in that said solid material (10) isa semiconductor.
 13. A source according to claim 12, characterized inthat said semiconductor is selected from the list constituted by:amorphous silicon, polycrystalline silicon, and GaN, GaAs, GaP, GaSb,InP, and derivatives thereof.
 14. A source according to claim 1,characterized in that it includes means for controlling the capture ofthe emitted photon by the probe (30).
 15. A source according to claim14, characterized in that said control means are means for controllingthe distance between the probe (30) and the light-emitting element. 16.A source according to claim 1, characterized in that said probe (30) isformed by a tapering point (31) of size smaller than 1 μm.
 17. A sourceaccording to claim 16, characterized in that said tapering point (31) ismade of a material that is dielectric or semiconductive.
 18. A sourceaccording to claim 16, characterized in that said tapering point (31) ismade of carbon or of silicon.
 19. A source according to claim 16,characterized in that said tapering tip (31) is constituted by the endof an optical fiber.